1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to an apparatus and method for generating a Medium Access Control (MAC) header with optimized information and interpreting the MAC header.
2. Description of the Related Art
Mobile communication systems are being developed to provide high-speed, high-quality wireless data packet communication systems that provide data service and multimedia service beyond the traditional voice service. A 3rd Generation (3G) mobile communication system, operating in Wideband Code Division Multiple Access (WCDMA), Universal Mobile Telecommunication Service (UMTS), which is based on the European systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), provides a uniform service of transmitting packet-based text, digitized audio or video data, and multimedia data at or above 2 Mbps to mobile phone users or computer users all over the world. The UMTS system enables access to any end in a network, relying on the concept of packet-switched access using a packet protocol such as Internet Protocol (IP).
The 3rd Generation Partnership Project (3GPP), working on UMTS standardization, is discussing Long Term Evolution (LTE) as a future-generation UMTS mobile communication system. LTE aims at high-speed packet communications at about 100 Mbps and for this purpose, many schemes are under discussion. Among them, there are techniques for reducing the number of nodes on a communication line by simplifying a network configuration or optimizing a wireless protocol to radio channels.
FIG. 1 illustrates a functionality of a MAC layer in a conventional LTE mobile communication system.
Referring to FIG. 1, in a transmitter, first and second Radio Link Control (RLC) entities 102 and 104 each construct an RLC Protocol Data Unit (PDU) with RLC Service Data Units (SDUs) received from an upper layer. A MAC layer 106 transmits the RLC PDUs to a receiver through a PHYsical (PHY) layer 108. In the receiver, a MAC layer 112 receives the RLC PDUs through a PHY layer 110 and provides them to their associated RLC entities 114 and 116. The RLC entities 114 and 116 extract the RLC SDUs and provide them to an upper layer.
The RLC PDUs are interpreted as MAC SDUs in the MAC layer 106. The MAC layer 106 generates a MAC header and configures a MAC PDU by concatenating the MAC header and the MAC SDUs. The MAC PDU may include MAC SDUs for control of transmission and reception between the MAC layers 106 and 112 of the transmitter and the receiver in addition to the MAC SDUs received from the RLC entities 102 and 104. The control MAC SDUs can be transmitted together with the data MAC SDUs in the MAC PDU or alone in the MAC PDU. Therefore, the header of the MAC PDU (referred to as the MAC header) is configured so that the control MAC SDUs can be distinguished from the data MAC SDUs.
FIG. 2A illustrates a MAC PDU format in a conventional mobile communication system.
Referring to FIG. 2A, a MAC PDU includes a MAC header 200 and Payloads 209, 210 and 211 each carrying one or more MAC SDUs. The MAC header 200 includes Logical Channel Identifications (LCIDs) 201, 204 and 207, Es 202, 205 and 208, and Lengths 203 and 206.
The LCIDs 201, 204 and 207 identify MAC SDUs delivered on different logical channels.
The Es 202, 205 and 208 each indicate whether there is another multiplexed MAC SDU. If an E is 0, this means that a MAC SDU corresponding to the E is the last MAC SDU. If an E is 1, this means that another multiplexed MAC SDU follows a MAC SDU corresponding to the E. In the latter case, a Length Field (LF) and an LCID follow the E.
The LFs 203 and 206 indicate the lengths of MAC SDUs corresponding to them. Hence, the LFs 203 and 206 should be long enough to represent the lengths of the MAC SDUs.
A minimum unit of multiplexed related information in a MAC header is referred to as a “subheader”. For instance, a subheader for a MAC SDU can be header information including an LCID, an E, and an LF. In some cases, part of the subheader may be omitted or one MAC header may include different types of subheaders. Although it is not a subheader for a MAC SDU, a later-described padding header is treated as a subheader carrying information about padding.
Referring to FIG. 2A, in the MAC header 200, a first subheader 213 with the LCID 201, the E 202, and the LF 203 relates to the first MAC SDU 209, a second subheader 215 with the LCID 204, the E 205, and the LF 206 relates to the second MAC SDU 210, and a third subheader 217 with the LCID 207 and the E 208 relates to the third MAC SDU 211.
As noted from FIG. 2A, the third subheader 217 does not include an LF because a MAC PDU size, i.e. a Transport Block (TB) size is known to both the transmitter and the receiver and thus the receiver can determine the size of the third MAC SDU 211 by subtracting the sum of the lengths of the LFs 203 and 206 and the length of the MAC header 200 from the TB size. Hence, an LF is not needed for the third MAC SDU 211 and instead, other user data can be transmitted in place of the LF. As a consequence, transmission efficiency is increased.
For accurate data transmission/reception, however, both the transmitter and the receiver should know which MAC SDU does not include an LF. For providing this information, two methods are usually considered.
In a first method, the transmitter notifies the receiver of the position of the absent LF. In a second method, the transmitter and the receiver preliminarily agree on the absence of an LF in a subheader at a particular position. Typically, an LF is absent in the last subheader.
However, the absence of the LF in the last subheader causes the following problem.
In general, a MAC PDU is padded. In padding the MAC PDU, padding bits (or dummy bits) are added in an empty space of a TB that has a smaller amount of transmission data than a predetermined TB size in order to match the TB to the predetermined TB size. To indicate the padding, a “padding subheader” is added to the header of the MAC PDU. If room still exists, payload is padded.
The absence of the last LF causes the following problem if the size of the padding is equal to or less than that of the last LF. Herein, the last LF refers to “the LF for the last MAC SDU” of one or more MAC SDUs included in a MAC PDU.
In the case where a TB size is set to 100 bytes and the last LF is 2 bytes, the absence of the last LF creates a 2-byte space empty in the TB, which is available for additional data transmission. If 1-byte of data is to be added, a necessary padding is 1 byte. If there is no additional data to be transmitted, a necessary padding is 2 bytes. In this case where the padding size is equal to or less than the size of the LF, the following contradiction is faced.
First, when the last LF is not included and 1-byte of data is added, 1-byte of padding is needed. To indicate the padding, a padding subheader is attached to the MAC header, which makes the last subheader no longer in the last place. Since the last LF is not the “LF for the last subheader”, it should not be omitted. Then the last 2-byte LF should be included in the MAC header, increasing the size of the TB to 102 bytes. Accordingly, the contradiction that the 1-byte of additional data and the 1-byte of padding should be deleted occurs.
Second, if the last LF is not included and there is no data to be added, a necessary padding is 2 bytes. This case also causes the above-described contradiction.